Such an underwater cutting type pelletizer includes a die (die for underwater cutting type pelletizer) attached to an end of a cylinder head of a mixer/extruder through a die holder; a water chamber provided on the downstream side of the die; a cutter driving shaft protruded into the water chamber; a cutter drive unit for rotating the cutter driving shaft; and a plurality of cutter knives fixed to the cutter driving shaft. The die has a number of die orifices. The water chamber has a water-cooling chamber which is filled with cooling water (hot water of about 60° C.) for cooling a thermoplastic resin extruded out of the mixer/extruder in such a manner that the cooling water can circulate (inflow and outflow). The cutter knives are held by a cutter holder fixed to an end of the cutter driving shaft to face a surface of the die.
In the thus-constituted underwater cutting type pelletizer, the thermoplastic resin supplied to the mixer/extruder is mixed by a screw, extruded into the water chamber in the form of strands through the die, and cut in the form of pellets by the cutter knives rotating on the surface of the die. In this way, underwater pelletization of the thermoplastic resin is performed.
A conventional die for underwater cutting type pelletizer to be used in this type of underwater cutting type pelletizers is shown, for example, in Patent Literature 1. A die for underwater cutting type pelletizer (hereinafter referred also simply to as a die body) of Patent Literature 1 aims at relaxing the thermal stress of the die body. The die body in Patent Literature 1 will be described in reference to FIG. 6. FIG. 6 is a perspective view showing a ⅛ segment structure of a conventional die for underwater cutting type pelletizer.
A die body 100 has a thick disk-like shape as the whole. The die body 100 includes a ring-like outer periphery fixing part 111, a die orifice part 112, an inner periphery fixing part 113 and a disk-like center part 115, which are disposed successively from the outer periphery toward the center. A number of outer periphery bolt holes are formed at circumferentially equal intervals in the outer periphery fixing part 111 so as to extend through from the front surface to the reverse surface in order to insert bolts for fixing the die body to a die holder.
A number of die orifices, not shown, for extruding strands, are formed in the die orifice part 112 so as to extend through from the reverse surface to the front surface. Further, a heating jacket, not shown, for distributing heated fluid (e.g., heated oil or steam) to the periphery of each die orifice (a passage through which the heated fluid flows) is formed within the die orifice part 112. The heated fluid is circulated to prevent the thermoplastic resin extruded from the mixer/extruder from being solidified within the die orifices. A plurality of inner periphery bolt holes is circumferentially formed in the inner periphery fixing part 113 so as to extend through from the front surface to the reverse surface.
In this die body 100, a temperature difference is caused between the die orifice part 112 increased in temperature by being heated by fluid of high temperature (about 300° C.) flowing in the heating jacket, and the inner periphery fixing part 113 and the center part 115 which are cooled by cooling water of about 60° C. within the water-cooling chamber of the water chamber. To reduce the thermal stress resulting from this temperature difference, the die body 100 has a cutout 114 including a slit 114b formed along the inner periphery on the front surface side of the inner periphery fixing part 113, as shown in FIG. 6.
In the die 100, a thermal stress generated on an outer periphery 113b of the inner periphery fixing part that is a boundary section between the die orifice part 112 and the inner periphery fixing part 113, namely a tensile stress generated in a radial direction is suppressed by the cutout 114, whereby the generation of damage on the die body 100 such as crack is prevented to attain an extended life of the die body 100.
However, in the die for underwater cutting type pelletizer (die body 100) of Patent Literature 1, an increased contact area between the cooling water in the water-cooling chamber of the water chamber and the inner periphery fixing part 113 due to the provision of the cutout 114 promotes the cooling of the die orifice part 112. In the die body 100 of Patent Literature 1, therefore, a new problem of solidification of the thermoplastic resin within the die orifices of the die orifice part 112 occurs. Further, the die body 100 may be deformed since the overall rigidity of the die body 100 is impaired.